1. Field of the Invention
The present invention relates to a belt transporting device that rotates an endless belt stretched around a plurality of stretching members, and to an image forming apparatus, such as an electrophotographic printer, copier, or the like, including the belt transporting device.
2. Description of the Related Art
Some of known image forming apparatuses such as electrophotographic printers and copiers each include a belt transporting device that rotates an endless belt stretched around a plurality of stretching members. It is known that such a belt transporting device has a problem in that, when the belt is rotated, the belt may be laterally shifted toward one end side in a belt width direction (the axial direction of the stretching rollers) that is orthogonal to the direction of belt rotation.
To solve the above problem, Japanese Patent Laid-Open No. 11-116089 discloses an exemplary device of adjusting the lateral shift of a belt by utilizing a force (running torque) acting in a direction of rotation of the belt. Specifically, the device disclosed by Japanese Patent Laid-Open No. 11-116089 includes contact members and cams. The contact members rotate in the direction of rotation of the belt by coming into contact with the belt that is under rotation. The initial positions (initial phases) of the contact members are determined by urging springs. The cams rotate together with the respective contact members. When the belt is laterally shifted toward one end side in the belt width direction, the belt comes into contact with one of the contact members, and a running torque is transmitted from the belt to that contact member. When the contact member rotates by a specific angle or larger, a corresponding one of the cams causes a corresponding one of the axial ends of a follower roller to be displaced in a direction vertical to the axis of the follower roller. Thus, in the device disclosed by Japanese Patent Laid-Open No. 11-116089, the lateral shift of the belt is adjusted by tilting the follower roller with respect to a driving roller.
The device disclosed by Japanese Patent Laid-Open No. 11-116089 includes the urging springs that urge the respective axial ends of the follower roller in the vertical direction so that the axial ends of the follower roller are returned to the respective initial positions. Specifically, after the lateral shift of the belt is adjusted, the follower roller is pulled by a corresponding one of the urging springs so that the tilt at the end of the follower roller is eliminated. Thus, the position of an axial end of the follower roller is adjusted.
That is, in the device disclosed by Japanese Patent Laid-Open No. 11-116089, to displace the axial end of the follower roller by rotating the contact member for adjusting the lateral shift of the belt, a running torque exceeding a restoring force exerted by the urging spring needs to be applied from the belt to the contact member. This may damage the end of the belt with a load applied thereto when the belt rotates the contact member. Moreover, while the axial end of the follower roller is being displaced, a force exceeding the spring force exerted by the urging spring is applied to an area of contact between the contact member and the belt. In such a situation, a large load is applied to the belt.